11. HIGH-RESOLUTION ROCK-MAGNETIC STUDY OF CEARA RISE SEDIMENTS AT SITE 9251

Peter A. Solheid,2 Subir K. Banerjee,2 Carl Richter,3 and Jean-Pierre Valet4

ABSTRACT

A variety of rock-magnetic measurements was made on sediment samples from Ceara Rise, Leg 154, Site 925 of the Ocean Drilling Program. Ceara Rise sediments are composed of marine carbonates and terrigenous material from the Amazon Fan. Bulk susceptibility, which measures approximately the concentration of strongly magnetic particles (e.g., magnetite, maghemite) in the sediment, can be used as a climate proxy signal that is based on the dilution of terrigenous material by ocean-derived carbonates. During interglacial periods, the high-susceptibility terrigenous material is diluted, resulting in a minimum in susceptibility. This hypothesis is supported by the inverse correlation between the smoothed susceptibility record and the low latitude record. Anhysteretic remanent magnetization (ARM), contributed primarily by magnetic grains from 0.1 to 15 Ám, was normalized by susceptibility (c), giving a parameter that is sensitive to magnetic grain size. Variations of ARM/c show high values during interglacials and low values during glacials. The source of this material is most likely from an increase of Amazon Basin soil runoff during interglacial periods.

1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Institute for Rock Magnetism, University of Minnesota, Minneapolis MN 55455-0128, U.S.A. peat@tc.umn.edu.
3Ocean Drilling Program, 1000 Discovery Drive, Texas A&M University, College Station, TX 77845-4857, U.S.A.
4Institut de Physique du Globe, 4 Place Jussieu, 75252 Paris Cedex 05, France.